The present invention relates generally to securing a sheet medium onto a support surface and, more particularly, to a method of and apparatus for firmly securing a flexible film sheet medium, in a preferred wrapped position, on a rotary drum so as to allow the sheet to be imprinted.
A wide variety of sheet processing systems have been proposed for effecting clamping of a sheet medium onto a cylindrical surface of a rotatable drum. For example, in facsimile machines, computer printers, and xerographic copiers, there are requirements for releasably clamping and wrapping a sheet medium to and about a rotary drum, whereby the medium can be imprinted while the drum is rotating. In general, the rotary drums of the above devices are rotated at relatively slow speeds, for example, in the order of about 10-100 rpms. However, with the advent of high speed digital dry laser imaging processes, such as the type commercially available from Polaroid Corporation of Cambridge, Mass., USA for use in obtaining high-quality radiographic images, there is a requirement that the film or medium be printed while being rotated at high speeds, such as in the order of about 1200 to 6000 rpm so that they can produce images within as commercially accepted time frames as conventional techniques. Another requirement is that the sheet being imaged remain in a preferred wrapped position for insuring the degree of image resolution required in the medical field. For example, one consequence of a sheet being misaligned or spaced from its desired wrapped position is that the quality of the resolution can be compromised significantly. This is especially critical with, for instance, radiological images of the medical type. In this regard, if the position of the film is off by as little as about .+-.40 microns from the intended plane, the resulting medical images are obviously less than the quality obtainable. To better appreciate the precision required in maintaining the sheet in its desired wrapped position, it should be considered that the thickness of a human hair is about 70 microns. Thus, it is evident that even minor deviations of a sheet from its intended wrapped position may cause unacceptable medical images.
The potential for a sheet deviating from its intended wrapped condition during digital imaging of the above type becomes even more significant whenever the size of the sheet to be printed increases. This is so because the larger format film sheets must be rotated at higher speeds so that they can be imprinted with considerably more information within the same general time frame as the smaller format sheets having less information. Because of increases in rotational speeds, there are increases in centrifugal forces acting on the sheet and the clamps. This tends to create problems with the sheet separating radially due to, for example, stretch of the sheet from its supporting drum and otherwise becoming misaligned, not to mention inducing clamping performance problems. These potential adverse effects of the centrifugal forces are even more pronounced when considering the fact that the centrifugal forces increase as the square of the increase of a drum's rotational speed. Furthermore, if the film sheets bulge or otherwise separate from the drum surface irregularly, then the printing laser head, which automatically moves toward and away from the sheet during printing in an effort to maintain the laser head at its desired focal plane distance to the print surface, will not be able to move in and out fast enough to maintain such desired focal distance. As a consequence, the rendered radiological images can be less then satisfactory.
One known approach for clamping a flexible sheet of dry laser imaging film onto a cylindrical surface of a rotatable drum, so as to be imprinted by a laser, is described in commonly assigned U.S. Pat. No. 4,903,957 issued Feb. 27, 1990. This patent discloses use of leading and trailing edge clamps which are mounted axially on a rotatable drum and are sequentially operated by external cams to clamp and release both the leading and trailing edges of the flexible sheet that is to be wrapped on a rotating drum.
Another known approach for clamping dry laser imaging film sheets to a rotary drum is present in a Helios 810 Laser Imager machine. The machine produces high quality 8.times.10-inch format radiographic images and is commercially available from the assignee of the present application. The clamping device employed clamps leading and trailing edges of a sheet to a cylindrical surface of a rotary drum. Each clamp is centrifugally actuated and has its center of gravity on one side of its pivot axis, whereby the center of gravity will pivot outwardly in response to centripetal acceleration forces, so as to provide corresponding and significant clamping forces directly radially inwardly on the medium by the clamp's claw.
While the foregoing approaches are satisfactory, there is nevertheless a desire to improve upon clamping performance, especially in situations wherein even high rotational speeds and centrifugal forces are to be encountered, such as when printing larger format film in the order of 14.times.17-inches as opposed to 8.times.10-inch film sheets.